Hello and welcome to Advanced Installer forums,
You can fully customize any predefined theme by using the Dialog Editor
from the Dialogs page.Here's a set of "Using the Dialog Editor" tutorials
to get you started. We also have a list of User Interface how to's
. There's always the website top right search field with which you can find relevant topics either in the user guide or forums.
For any other related doubts or queries we're here to help.
All the best,